pub fn dp_rec_mc(mut amount: u32) -> u32 {
    let dollars = [100, 50, 30, 20, 10, 5, 2, 1];  // 硬币面值, 由大到小
    let mut num: u32 = 0;
    for dollar in dollars {  // 从大面值到小面值找零
        num += amount / dollar;
        amount %= dollar;
    }
    num
}
